Zeolites are crystalline aluminosilicates with a regular three-dimensional porous lattice structure built up from SiO4″ and AlO4″ tetrahedra in which the negative charges are compensated by mono or multivalent cations. These cations are exchangeable without the lattice structure being destroyed. Their use as water softener is mainly attributed to this property. Hard water contains calcium and/or magnesium salts, which greatly reduce the surfactant effect of soaps and detergents. The exchange of sodium ions present in the zeolite for the calcium and magnesium ions present in the water softens the hard water. Generally, phosphates are used as builder in household detergents. Due to the increasing awareness about the polluting effect and non-biodegradable property of phosphates, many countries have banned or curtailed the use of phosphates in household detergents. Zeolite-A is established as the most suitable substitute for phosphate builder in detergents. Under the circumstances, use of Zeolite-A is increasing as detergent builder.
Zeolites are inorganic materials having high thermal and hydrothermal stability. These materials are also chemically stable at ambient temperatures towards may organic compounds. Zeolites owing to the presence of molecular dimensional pores are also used as molecular sieves and as adsorbents for drying, purification and separation of compounds. They have significant adsorption capacity of water even at very low partial pressures and hence are effective desiccants, with a capacity of up to more than 25% of their weight. They are also used to remove volatile organic chemicals from air streams, separate isomers and mixtures of gases.
Zeolites can act as shape-selective catalysts either by transition state selectivity or by exclusion of competing reactants on the basis of molecular diameter. Zeolites have also emerged as solid acid catalysts and have substituted conventional acids like sulphuric acid in many applications. A variety of organic transformations, namely, alkylation, acylation, isomerisations, oxidation is being carried out employing zeolite based catalysts where zeolite acts as a catalyst or catalyst support. The industrial sectors where zeolites have made substantial impact as catalysts and adsorbents include: petroleum refining, synfuels production, and petrochemical production.
The largest volume wise application of zeolite is in detergent industry with Zeolite-A being used as detergent builder. The specific properties which make Zeolite-A suitable as phosphate substitute detergent builder include:                The high cation exchange capacity even at higher temperature makes Zeolite-A effective in removal of water hardness ions, particularly calcium.        It gives alkaline reaction in aqueous medium with pH less than 12.        It does not cause encrustation on the fabric.        Detergent grade Zeolite-A crystals are cubic in shape with rounded corners and edges and can pass through the mesh of the fabrics allowing easy removal during rinsing.        The surfactant adsorption capacity of the Zeolite-A is several times higher than the polyphosphates.        Zeolite-A absorbs unwanted water-soluble molecules from the dirt.        It coagulates the colloidal dirt particles and pigments causing easy removal from the aqueous phase.        It does not clog the sewerage.        It does not exert any negative influence upon biological sewerage purification.        It does not remobilize heavy metals.        Zeolite-A is toxicologically innocuous.        
Conventionally, Zeolite-A is synthesized using aluminum and silicon rich material as the stating materials in the presence of au alkali. Aluminum trihydrate, Aluminum alkoxide, and Sodium aluminate are used as aluminum source and fume silica, sodium silicate and colloidal silica are employed as silica source. Mixing solutions of silicate and aluminate produce aluminosilicate gel that precipitates. The gel thus formed is then crystallized to Zeolite-A by aging in the mother liquor at higher temperature. Such processes are described; for example, in U.S. Pat. Nos. 2,841,471 and 2,847,280 (1958), and in French Patent 1,404,467.
The cost of production of Zeolite-A depends largely on the starting materials used, especially, for the detergent builder Zeolite-A, where the volumes employed are very large. Since conventional detergent builders like sodium tripolyphosphate (STPP) are economically attractive compared to Zeolite-A, in many countries there is a resistance to substitute STPP by Zeolite-A in spite of eco-friendly nature of the latter. Therefore, research efforts are directed towards developing a process for Zeolite-A synthesis wherein the Zeolite-A developed can be economically competitive to STPP.
Kimberlite tailings are produced as a huge solid waste during diamond mining. For example, in a country like India from its Panna diamond mines, typically around 100 tones of Kimberlite is generated per 10 carat of diamond mined. Around 3-4 million tones of Kimberlite is already accumulated during previous diamond mining in India. With an estimated life of 20 years for the Panna diamond mines, huge quantity of Kimberlite tailing is likely to be available in the country. Considering this problem on the global scale, as countries like South Africa and Canada are also involved in diamond mining, the quantity of Kimberlite tailings to be generated is of a serious concern. Therefore, it is pertinent to look for technical solutions to gainfully utilize Kimberlite tailings accumulated during diamond mining. The Kimberlite being rich in magnesia and silica, there is an opportunity to develop magnesium and silica based products from this materials. Therefore, efforts were made to prepare value-added product like Zeolite-A. The value-addition of Kimberlite will not only make diamond mining process more economical but will also make it an eco-friendly. Kimberlite with a typical chemical composition given below is rich source of silica and therefore considered as a potential starting material for the preparation of Zeolite-A.
The typical chemical composition of Kimberlite is given below:    SiO230-32%, Al2O32-5%, TiO25-8%, CaO 8-10%, MgO 20-24%, Fe2O35-11%, LOI 13-15%.
U.S. Pat. No. 3,101,251 (1963) discloses a process for producing Zeolite-A wherein a non-kaolinitic aluminosilicate in admixture with an alkali metal hydroxide is fused at a temperature from 330 to 370° C. An aqueous reaction mixture is formed with this fused admixture. This reaction mixture has water to sodium oxide molar ratio of from 35:1 to 200:1, a sodium oxide to silica molar ratio of from 1.3:1 to 2, 5:1, and silica to alumina molar ratio of from 0.8:1 to 3:1. This reaction mixture is reacted at a temperature from 20 to 120° C. until Zeolite-A is formed. The process has limitations of high temperature alkali fusion of aluminum and silica contain solids prior to crystallization.
U.S. Pat. No. 5,965,105 (1999) disclosed a process for the synthesis of Zeolite-A using fly ash. In this process, fission mixture was obtained by mixing fly ash and caustic soda in a ratio of 1:1.2 and optionally adding sodium aluminate or aluminium hydroxide. This fusion mixture was heated at 500-600° C. for about 1-2 hours, to obtain a fused mass. This fused mass was treated with distilled water for 8-10 hours with simultaneous optional addition of sodium aluminate or alum solution, in the presence or absence of NaCl, followed by optional addition of Zeolite-A seeds to obtain amorphous aluminosilicate slurry; subjecting said slurry to hydrothermal crystallization at about 90-110° C. for 2 to 4 hours to obtain Zeolite-A crystals. However, this process requires very high reaction temperature and high processing time in cooling, milling and mixing, moreover, seeding of Zeolite-A crystal is also necessary. The process has limitation of fusion of fly ash and caustic soda at a high temperature of 500-600° C. making the process energy intensive.
Production of high purity fine size Zeolite-A by adding organic acid to silica and/or alumina-silica, sodium aluminate and sodium hydroxide is described in Japanese Patent 54,081,200, (1997). The limitation of this patent is that the addition of organic acid to silica and alumina source is required.
U.S. Pat. No. 4,089,929 describes use of mineral aluminosilicate raw materials to produce low-iron zeolitic-aluminosilicates. Methods for preparation and use of zeolite containing cation exchanger using expanded clays/ceramics are described in U.S. Pat. No. 5,976,490. These processes include calcination of raw materials which is an energy intensive process. Calcined material is treated further with alkali solution to produce desired product.
U.S. Pat. No. 4,405,484 (1983), describes a process for preparing zeolite powder having high flow ability i.e. general flow index (expressed as the sum of indexes of the repose angle, spatula angle, compressibility and degree of cohesiveness) of from 30 to about 50 and at least 99 weight % of particles are having a particle size of 1-5 microns. The zeolite powder is prepared by adding an alkali metal aluminate to an aqueous zeolite slurry containing 30-52 wt. % of zeolite (as anhydride) and having pH of not higher to 12.8; adjusting the pH of the slurry to a value not higher than 11; and then, drying the slurry. The zeolite powder is useful as a builder for detergent. The process requires pH adjustment and zeolite slurry.
Indian Patents 182635 and 182636 describe improved processes for preparing aluminosilicate gel and manufacture of detergent grade zeolite from it under milder conditions of temperature and time. However, the use of commercially available sodium aluminate powder as a source of alumina is contributing much to the production cost.
Wantae Kim et al. (Journal of Chemical Engineering of Japan, Vol. 33, No. 2, pp. 217-222, 2000) investigated a novel process for synthesizing Zeolite-A and X from kaolinite activated by dry grinding. The process consists of grinding of kaolinite and subsequent reaction with NaOH solution at 60° C. Zeolite-A and X can be synthesized from the process under normal pressure. Crystallization of Zeolite-A and X is influenced by the activated state of kaolinite. The drawback associated with this is the requirement of grinding which is an energy intensive process.
U.S. Pat. No. 6,641,796 describes a method for making zeolites and zeolite mixtures having enhanced cation exchange properties i.e. >200 mg CaCO3/gram anhydrous zeolite. This method include mixing a sodium silicate solution, a sodium aluminate solution, and an amorphous aluminosilicate initiator got in a mixing vessel to create an aluminosilicate synthesis get and crystallizing the aluminosilicate synthesis gel to form zeolite crystals. The drawback of this method is the requirement of initiator gel to produce aluminosilicate synthesis gel.
U.S. Pat. No. 6,773,693 describes preparation of fine. A-type zeolite particle having an average primary particle size of 0.1 mμm or less. The process for preparing the fine A-type zeolite particle comprise reacting a silica source with an aluminum source in presence of an organic compound having an oxygen-containing functional group and a molecular weight of 100 or more. Requirement of adding organic compound is a limitation of this process.